As illustrated in FIGS. 22A to 22C, a sheet detecting device including a flag 223 and a sensor 224 for detecting a sheet conveyed through a pair of sheet conveying rollers 218 and 219 is disposed downstream of the pair of sheet conveying rollers 218 and 219 in a sheet conveying direction.
The flag 223 includes a shaft 227 which serves as the center of rotation of the flag 223 and a light-shielding member 225 which shields a light path from a light-emitting portion to a photo detector in the sensor 224. The flag 223 further includes a stopper portion 226. As illustrated in FIG. 22A, the flag 223 is urged clockwise by a spring or the like. The stopper portion 226 of the flag 223 is in contact with a stopper 226a of an apparatus frame, thereby restricting the rotation of the flag 223. Thus, the flag 223 is held in a standby position.
As illustrated in FIG. 22B, when the leading edge of a sheet, conveyed through the pair of sheet conveying rollers 218 and 219, abuts a contact surface 223a of the flag 223, the flag 223 begins swinging about the shaft 227 in the direction, indicated by the arrow in FIG. 22B, from the standby position. As illustrated in FIG. 22C, the light-shielding member 225 shields the light path from light and the sensor 224 detects the light-shielding and outputs a signal. On the basis of this signal, the sheet detecting device detects that the leading edge of the sheet has been conveyed to an area corresponding to the flag 223. When the trailing edge of the sheet passes the area corresponding to the flag 223, the flag 223 again swings to the standby position illustrated in FIG. 22A and is ready to detect the next sheet.
In other words, the flag 223 reciprocates between the standby position and a position where the flag 223 pressed by a sheet allows the sheet to pass each time the sheet passes (refer to Patent Literatures 1 and 2).
A result of detection by the above-described sheet detecting device is used as follows, for example. In an image forming apparatus for forming an image on a sheet, the timing when a sheet conveying unit conveys a sheet to an image transfer unit is adjusted on the basis of the result of detection by the sheet detecting device so that an image formed by an image forming unit is formed in a predetermined position of the sheet. The timing when the image forming unit starts image formation is adjusted on the basis of the result of detection by the sheet detecting device so that an image formed by the image forming unit is formed in the predetermined position of the sheet. In addition, the result of detection by the sheet detecting device is used to detect, for example, a delay in sheet conveyance or a jam in a sheet conveying path.
CITATION LIST
Patent Literature
PTL 1 Japanese Patent Laid-Open No. 6-94444
PTL 2 Japanese Patent Laid-Open No. 10-114446
In response to user demands for further increased productivity (the number of image-formed sheets per unit time) of the image forming apparatus, an increase of sheet conveying speed or a reduction of the interval (hereinafter, referred to as “sheet interval”) between the trailing edge of a preceding sheet and the leading edge of a succeeding sheet is being desired. Accordingly, the flag is required to again return to the standby position for aligning the leading edge of the succeeding sheet in a short sheet interval after the trailing edge of the preceding sheet passes.
As described above, in the related-art sheet detecting device, the flag reciprocates each time a sheet passes. Therefore, the following distance is needed as a minimum distance required as the sheet interval. A distance D1 is set as a distance in which the contact surface 223a of the flag 223 returns from the position of the contact surface 223a located when the trailing edge of the preceding sheet passes the contact surface 223a of the flag 223, as illustrated in FIG. 22C, to the standby position where the contact surface 223a aligns the leading edge of the succeeding sheet, as illustrated in FIG. 22A. A distance D2 is set as a distance where the succeeding sheet is conveyed while the contact surface 223a returns from the position of the contact surface 223a located when the trailing edge of the preceding sheet passes the contact surface 223a of the flag 223 to the standby position. The minimum distance required as the sheet interval between the preceding sheet and the succeeding sheet is a distance D3 (D1+D2=D3) obtained by adding the distance D1 and the distance D2. Specifically, when the sheet interval is shorter than this distance, the succeeding sheet reaches the standby position before the contact surface 223a of the flag 223 returns to the standby position. Disadvantageously, the sheet cannot be detected.
To increase the productivity of the image forming apparatus, the sheet conveying speed may be increased in addition to the reduction of the sheet interval. However, the increase of the sheet conveying speed causes the following problem.
The distance D2 in which the succeeding sheet is conveyed during a returning operation of the flag is calculated by multiplying the time ΔT during which the flag 223 returns from the position illustrated in FIG. 22C to the standby position in FIG. 22A while rotating in the direction opposite to the sheet conveying direction by a sheet conveying speed V (ΔT×V=D2). Accordingly, the higher the sheet conveying speed, the longer the distance D2 needed. As described above, as the sheet conveying speed is increased, the minimum distance required as the sheet interval has to be set longer. It is difficult to substantially increase the productivity.
In the sheet detecting device using the reciprocating flag, therefore, the increase of the productivity (the number of conveyed sheets per unit time) related to sheet conveyance is restricted because it is limited by the time for return of the flag.